In broad terms, electrosurgery is the application of a high-voltage, high-frequency (HF) or radio-frequency (RF) output waveform to tissue to achieve a surgical effect. Tissue is cut, coagulated by stopping blood flow, or simultaneously cut and coagulated, depending upon the characteristics of the electrosurgical output signal. To achieve cutting, the output signal is substantially continuous. To achieve coagulation, the output signal is delivered in bursts with each burst defined by a duty cycle in which the on-time of the duty cycle is substantially less in time duration than the off-time. To achieve simultaneous cutting and coagulation, the output signal is also delivered in bursts, but the on-time and the off-time of the duty cycle are comparable in time to each other, or the on-time may exceed the off-time. The electrosurgical output signal is delivered to the tissue from an active electrode of an applicator or handpiece that is manipulated by the surgeon. The output signal is conducted to the electrode of the applicator over a conductor extending from the electrosurgical generator to the applicator or handpiece.
The load into which the electrosurgical output signal is delivered varies substantially during a surgical procedure due to large and almost instantaneous changes in the point-to-point resistance or impedance of the tissue encountered. For example, a highly fluid-perfused tissue, such as the liver, may exhibit a resistance or impedance in the neighborhood of 10-20 ohms while other tissues, such as skin or bone marrow, may have an impedance in the neighborhood of 1000 to 2000 ohms. When the active electrode passes from low impedance tissue into high impedance tissue, less current is momentarily delivered to the high impedance tissue thereby immediately degrading or inhibiting the desired electrosurgical effect. On the other hand, when the active electrode passes from high impedance tissue into low impedance tissue, high current is momentarily delivered into the low impedance tissue and a high current may create excess tissue damage. The variable impedance characteristics of the tissue require the electrosurgical generator to deliver and control relatively wide variations of power on essentially an instantaneously changing basis.
The practical effects of load variations resulting from the rapidly changing tissue resistance or impedance and the need to regulate a high-frequency, high-voltage electrosurgical output signal, create substantial limitations on the performance of an electrosurgical generator. If the control system of the electrosurgical generator cannot respond to the rapidly changing conditions encountered during electrosurgery, the output power regulation may not be sufficient to avoid unintended effects. Signals supplied by sensors of the electrosurgical output signal may not be processed quickly enough to be of effective use in regulating the output power. A control loop time lag or phase lag, which is that time between acquiring the sensed signals and making an adjustment in the output signals, maybe so long that a response cannot be achieved quickly enough to obtain or maintain the desired effect. The control loop time or phase lag is dependent upon many factors, but a principal factor relates to the speed at which the output voltage and current signals may be derived and processed into usable feedback and output control signals. The same circumstance also applies with respect to monitoring other output-related factors, such as tissue impedance, which must be calculated based on the instantaneous values of output voltage and current signals.
In addition to a rapid response time, the most effective control system for an electrosurgical generator should recognize the difference between real power and reactive or imaginary power. Real power produced by an electrosurgical generator creates the electrosurgical effect, while reactive power has no immediate electrosurgical effect. Reactive power is a consequence of the capacitive or inductive reactance of the entire system, principally including the output circuit to and from the patient.
If an electrosurgical generator uses a power feedback control system, a common approach to regulating output power is based on apparent power, rather than real power. Apparent power is the vector sum of the real and reactive power. Reactive power contributes to apparent power, but reactive power does not create the electrosurgical effect. Real power represents what can be expected as the electrosurgical effect, and apparent power is always more than the real power because of the reactive or imaginary contribution to apparent power. The difference between the power expected and the power delivered during electrosurgery can be substantial and important in achieving a satisfactory electrosurgical effect.
Distinguishing between real and apparent power requires knowledge of accurate output voltage and current values, and the relation or phase angle of the output voltage and current waveforms. Most typical electrosurgical generators do not have the capability to acquire or process such phase angle information, because to do so involves a complex control system with a fast measuring system. Moreover, the components of many control systems and the functionality of those control systems cannot perform or respond quickly enough to provide the necessary information to distinguish between real and apparent power. Indeed, many electrosurgical generators are open ended, and as such, have no capability to regulate output power using feedback.
A feedback control system based upon apparent power can sometimes degrade electrosurgical effects. For example, in endoscopic applications where a substantial amount of capacitance exists due to conducting the electrosurgical output signal within a relatively long endoscope, a significant portion of the apparent delivered power will be reactive or imaginary power. The substantial capacitance created by the endoscope must be charged with power and that component of the output power becomes reactive or imaginary. The diminished real power component of the output power might be insufficient to achieve the desired surgical effect. Another example involves the situation where both the apparent power and the real power are below the desired power output selected by the surgeon. In this situation, as the control system increases the power to the desired output power, because apparent power is greater in magnitude than real power, the control system will fail to ever deliver enough real power. In still other cases involving patient circuits with a high amount of reactance, such as minimally invasive procedures where the electrosurgical instruments are inserted inside of an endoscope or a laparoscope, regulation on the basis of apparent power may in some cases actually result in the delivery of more than the desired amount of power. The stored reactive power may be delivered as real power at unexpected times. In those open ended electrosurgical generators which have no feedback control, any load reactance is one more energy storage component which must be charged. Storing the added reactance with energy adds to the potential that the reactance will deliver that added power under unexpected circumstances. These and other exemplary cases of failing to distinguish between apparent power and real power during electrosurgery raise the risk of unintended surgical effects, diminished effectiveness of the surgical effect, and longer times required to complete the surgical procedure.